Devices for collecting airborne particles (airborne particle collectors) are now on the market. Among them are those that utilize impaction, and such airborne particle collectors have a high capture rate of microparticles and are now receiving attention.
Impaction refers to a method of impacting particle-containing air through nozzles onto a capture surface and thereby attaching the particles to the surface.
Conditions for collecting particles by impaction are expressed by the Stokes number Sk given below.Sk=pd2uC/9μw  (1)where w is the diameter of the nozzles, u is the average air speed in the nozzles, μ is the viscosity coefficient of the air, p is the density of the particles, d is the diameter of the particles, and C is the Cunningham correction factor (the correction factor for particle movements, about 1.15 under atmospheric pressure).
According to “Aerosol Technology: the Critical Particle Diameter and the Collection Efficiency of Impactors,” (Inoue Shoin, April 1985), when the square root of the Stokes number Sk of the above formula (1) is 0.7 or higher, nearly 100% of the particles will collide with the capture surface.
Airborne microorganism collecting devices that utilize impaction can be classified into two types: those designed for collecting bacteria (minimum particle size=about 1 μm) and those designed for collecting aerosols (minimum particle size=several tens of micrometers).
Examples of the airborne microorganism collecting devices designed for collecting bacteria include the portable airborne bacteria sampler disclosed in Patent Document 1 and the air sampler and sampling method for microbiological analysis disclosed in Patent Document 2 (see below).
Those devices collect bacteria by impacting bacteria-containing air from multiple nozzles onto a culture medium (i.e., a capture surface). The air impacted onto the capture surface then flows through the space between the culture medium and the nozzle-including plate toward the circumferentially outer side of the capture surface.
An example of devices for collecting aerosols (minimum particle size=several tens of micrometers, which is much smaller than that of bacteria) is the low pressure impactor disclosed in Non-Patent Document 1. This impactor uses a vacuum pump to suction air. By the vacuum pump greatly lowering the exit-side pressure of the nozzles, the Cunningham correction factor C can be increased. As a result, the Stokes number of Formula (1) is also increased, allowing capture of very small particles.
The low pressure impactor of Patent Document 3 is designed such that when air is impacted from the nozzles onto the capture surface, the air flows toward the air vent located at the center of the capture surface.